Add README for onlyargs_derive (#3)

onlyargs_derive/README.md

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+[![Crates.io](https://img.shields.io/crates/v/onlyargs_derive)](https://crates.io/crates/onlyargs_derive "Crates.io version")
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+
+Only argument parsing! Nothing more.
+
+# Why?
+
+- 100% safe Rust 🦀.
+- Correctness: Paths with invalid UTF-8 work correctly on all platforms.
+- Fast compile times.
+  - See [`myn` benchmark results](https://github.com/parasyte/myn/blob/main/benchmarks.md).
+- Convenience: `#[derive(OnlyArgs)]` on a struct and parse CLI arguments from the environment into it with minimal boilerplate.
+
+## MSRV Policy
+
+The Minimum Supported Rust Version for `onlyargs` will always be made available in the [MSRV.md](../MSRV.md) file on GitHub.
+
+# Examples
+
+See the [`derive-example`](../examples/derive/src/main.rs) for usage.